The present invention relates in general to an improved fluid-filled bushing structure for vibration damping or isolation, and more particularly to a highly durable bushing structure which includes a resilient member and contains a non-compressible fluid to effectively utilize a hydraulic damping effect of the fluid as well as to provide vibration absorbing capability by means of the resilient member.
In the art of a suspension system for use, for example, on an automotive vehicle, various types of suspension members in the form of arms, rods, links, etc., are connected to the vehicle chassis to suspend a differential gear, wheels, and similar parts of the vehicle so that the suspension members are pivotable in various directions. To damp or absorb vibrations, suspension dampers or isolators in the form of bushings have been commonly used as pivotal connections at both ends of each suspension member. It is also known to use similar bushings as engine mounts which are interposed between an engine and the vehicle chassis to restrain transmission of vibrations from the engine.
Generally, a bushing structure serving as a vibration damper or shock absorber like such suspension bushing or engine mount as stated above, comprises an inner metal sleeve through which is inserted a suitable flucrum shaft for suspension purpose of the vehicle, an outer metal sleeve disposed concentrically with the inner metal sleeve, and an annular resilient or elastomeric member radially compressed between the inner and outer metal members for damping vibrations through deformation or compression thereof. In recent years, however, it has been proposed to employ a compound bushing structure, i.e., a fluid-filled bushing which is capable of effecting a required vibration damping without having to use a rubber or resilient material of particularly high damping coefficient for the resilient member.
More specifically described, as disclosed in U.S. Pat. Nos. 3,642,268 and 3,698,703 and Japanese Patent Aplication TOKU-KAI-SHO No. 56-164242 (laid open for public inspection), a resilient member interposed between the inner and outer metal sleeves of such compound bushing as stated above has a plurality of recesses which cooperate with the inner wall surface of the outer metal sleeve to form pockets or cavities which are filled with a suitable fluid and arranged to be in fluid communication with each other through an orifice. The fluid flows through the orifice from one of the pockets to another upon transmission of vibrations to the bushing, and thus the orifice provides a resistance to the fluid flow which gives a good damping effect.
While such compound bushing structure known in the art is able to provide a desired fluid damping force by means of a resistance (viscosity resistance) to the fluid flow through the orifice, no provisions are made for protection against excessive deformation or deflection of the bushing structure, that is, of the resilient member or rubber insert inserted between the inner and outer cylindrical members. Under these conditions, the resilient member is deformed according to the magnitude of a load applied thereto through the outer cylindrical member. Upon application of an extremely high load, the resilient member may have an excessive deformation (strain), and tends to be damaged due to repetitive deformation thereof in such excessive degree. Thus, the prior compound bushing structure suffers a problem of low durability or short life. In addition, the known compound structure has another potential problem that, in the event of application of a high load to one part of the bushing, another part thereof diametrically opposite to said one part becomes unable to deform in such degree as to follow the magnitude of the excessive load applied, thereby causing reduction in its function of sealing with respect to the inner surface of the outer metal sleeve and thus inducing the possibility of external leakage of the fluid contained within the pockets.